134 



NATURE OF PLANT VARIATIONS 



The diagram, Fig. 86, A, shows more clearly the results of 

 crossing discussed above. The black circles represent a plant 

 bearing black seeds and the unshaded circles one with yellow seeds. 

 Each of these plants produces gametes containing black and white 

 unit characters respectively. By the fusion of these gametes a 



Parents with black and yellow 

 seeds respectively. 



Gametes. 



First generation, hybrids. 



Gametes, black and yellow 

 unit characters equally seg- 

 regated. 



Second generation, }/^ pure 

 stock, 3^ hybrids. 



Fig. 86a. 



hybrid is produced containing both unit characters, but the seeds 

 appear black because of the dominance of this color. When 

 hybrids produce gametes the unit characters are segregated so 

 that each gamete represents a pure strain, in this case either 

 black or yellow. In other words, the gametes from a hybrid are 

 as pure as those derived from the original stock. Since each 

 hybrid plant produces two kinds of gametes there are four pos- 

 sible combinations of them: (i) Black with black forming pure 

 black, (2) white with white forming pure white, (3) black with 

 white forming a hybrid with black dominant, (4) white with 

 black forming a hybrid with black dominant. This diagram 

 brings out graphically what is meant by pure stock. We com- 

 monly consider a stock as pure bred for any character or charac- 

 ters when its ancestors for several generations — the longer the 



